1. Field of the Invention
The present invention relates generally to a fluid-filled resilient or elastic bushing, and more particularly to an improved vibration damping and isolating resilient bushing for automotive vehicles, which is filled with a viscous fluid and utilizes a viscosity resistance of the fluid exhibited upon application of shearing stresses to the fluid masses.
2. Discussion of the Prior Art
There are known various resilient vibration damping and isolating mounting or bushing structures, such as body mounts, cab mounts, cross-member mounts, strut-bar cushions, tension-rod bushings, suspension arm bushings, and engine-roll stoppers for FF vehicles. These resilient mounting or bushing structures are required to effectively absorb or damp a vibrational energy where the input vibrational load has large amplitude, like vibrations as produced upon a sudden start of a vehicle, upon application of an abrupt brake to the vehicle, or during shaking movements of its engine. Further, the structures are required to damp continuous resonance vibrations, for reducing a force to be transmitted therethrough. The structures are also required to isolate high-frequency vibrations having a comparatively small amplitude, which are generated during normal operation of the engine or due to irregularities of the road surface. In short, these resilient mounting or bushing structures are generally required to exhibit high damping capability for low-frequency vibrations having a large amplitude, and a relatively low dynamic spring rate or constant for high-frequency vibrations having a small amplitude.
Usually, the resilient mounting or bushing structures employ a solid rubbery or elastomeric material. Accordingly, a resilient structure adapted to provide high damping characteristics tends to have a high dynamic spring constant for high-frequency vibrations having a small amplitude. Conversely, if a resilient structure is adapted to provide a low dynamic spring constant for high-frequency vibrations without changing the static spring constant, the resilient structure necessarily exhibits low damping characteristics. Therefore, the mere selection of a suitable rubbery material does not permit such resilient structures to provide two different characteristics, that is, high damping capability for low-frequency vibrations, and high isolating capability for high-frequency vibrations.
In the light of the above drawback, a fluid-filled resilient mounting or bushing structure has been proposed in recent years. An example of such a fluid-filled resilient structure is disclosed in U.S. Pat. No. 3,642,268 and U.S. Pat. No. 3,698,703. This fluid-filled structure has two fluid chambers filled with an incompressible fluid. These chambers communicate with each other through an orifice provided therebetween. Upon application of a vibrational load to the structure, the volume of one of the two fluid chambers is reduced, and the fluid is forced to flow through the orifice from that one fluid chamber to the other. In this type of resilient mounting or bushing structure, the input low-frequency vibrations can be effectively damped due to inertia and resonance of the fluid mass in the orifice when the fluid is forced to flow through the orifice. The frequency range of the vibrations to be damped can be selected by suitably dimensioning the orifice.
If the orifice connecting the two fluid chambers of this type of fluid-filled structure is dimensioned (in terms of its length and cross sectional area or diameter) so as to provide excellent damping characteristics for low-frequency vibrations, then the vibration isolating capability of the structure is accordingly reduced for vibrations having comparatively high frequencies and small amplitudes. In other words, the resilient structure is too stiff to effectively isolate high-frequency vibrations. Thus, there has been a need to develop a fluid-filled resilient mounting or bushing structure which is satisfactory in the overall vibration damping and isolating capability or characteristics.